An organic electroluminescent (hereinafter abbreviated to “EL” when appropriate) device is a planar light emitting device having a structure such that an organic thin film is sandwiched between a pair of electrodes, and is characterized as being thin and light-weight and having a wide view angle and a high speed response, therefore, is expected to be suitable for various display devices. Recently organic EL devices have been practically applied as a display for personal digital assistants such as a cell phone.
The organic EL device utilizes light emission occurring due to the recombination of electron injected from a cathode with hole injected from an anode, which occurs in the emitting layer. Most organic EL devices have a multilayer structure comprising a hole transport layer, an emitting layer and an electron transport layer. Charge transport layers such as the hole transport layer and the electron transport layer themselves do not emit light, but have a function of promoting injection of an electron into the emitting layer and confining electrons injected into the emitting layer and energies of an exciton formed in the emitting layer. Thus, the electron transport layer plays an important role for improving low-voltage drivability and emitting efficiency of organic EL devices.
As a hole transport material, an amine compound having an appropriate ionization potential and a hole-transportability is used. Such hole transport material includes, for example, well-known 4,4′-bis[N-(1-naphthyl)-N-phenyl]biphenyl (hereinafter abbreviated to as “NPD”). However, an organic EL device having a hole transport layer comprising NPD does not exhibit driving voltage and emitting efficiency to a satisfying extent, and hence, a novel hole transport material is eagerly desired.
In recent years, an organic EL device having an emitting layer comprising a phosphorescent material has been developed. A hole transport material having a high triplet level is required for the organic EL device using a phosphorescent material. From a viewpoint of triplet level, NPD is not sufficient. For example, it is known that an organic EL device using a combination of NPD with a phosphorescent material emitting a green light exhibits a low emitting efficiency (see non-patent document 1, below).
In view of the above-mentioned background, it has been recently proposed to utilize a compound having an aromatic cyclic hydrocarbon group having introduced therein carbazole ring(s), specifically, a 3-aminocarbazole compound (see for example, patent documents 1 and 2) and a 2-aminocarbazole compound (see for example, patent documents 3 and 4).
However, the measurement of the 3-amoinocarbazole compound by the inventors revealed that their ionization potentials are lower than that of NPD although the compound have a higher triplet level than that of NPD. It is presumed that the low ionization potential of the 3-aminocarbazole compounds is due to the fact that the amino group in the carbazole ring is activated by the nitrogen atom at 9-position of the carbazole ring.
Ionization potential of a material plays an important role for the characteristics of an organic EL device. Especially when the material is used in a hole transport layer adjacent to an emitting layer, the ionization potential of the material greatly influences upon emitting efficiency of an organic EL device. For example, when a material having a low ionization potential is used in a hole transport layer adjacent to an emitting layer, the emitting efficiency of an organic EL device is greatly reduced by the exciplex formed between the hole transport layer and the emitting layer (see, for example, non-patent document 2). Therefore, an organic EL device having a hole transport layer comprised of 3-aminocarbazole compound does not exhibit a sufficiently high emitting efficiency.
The 2-aminocarbazole compound has an ionization potential equal to or higher than that of NPD, and a triplet level higher than that of NPD. In organic EL devices using a green phosphorus luminescent material, the 2-aminocarbazole compound exhibits a higher emitting efficiency than that of NPD. But, a material exhibiting a still higher emitting efficiency is eagerly desired.